It has been recognized for over twenty years that the kidney is able to escape from the antidiuretic effect of the hormone vasopressin (VP) following chronic VP administration. Since the mechanism underlying this escape phenomena has not been delineated, the objective of the present proposal is to determine the factor(s) responsible for development of renal resistance to the antidiuretic effect of chronic VP. During the past year, we observed that VP escape occurred despite unchanged renal medullary solute concentrations, glomerular filtration rate or osmolar excretion. Since there is substantial evidence that the 3',5'-cyclic adenosinemonophosphate (cAMP) system mediates in part the antidiuretic effect of VP, analyses of adenylate cyclase, cAMP, phosphodiesterase and protein kinase in renal tubular tissue removed in vivo from animal models of VP escape will be undertaken. If these studies indicate a role for the cAMP system in VP escape, then in vitro studies will be undertaken in renal tissue slices and isolated collecting ducts to determine the mechanism for cAMP system abnormalities. Substantial evidence suggests that VP stimulates renal prostaglandin (PG) synthesis and that PG's are capable of antagonizing the hydroosmotic effect of VP. Moreover, in our model of VP escape, early escape was associated with a significant increase in urinary prostaglanding excretion. Thus a role of PG's in VP escape will be assessed by analyzing the effect of PG inhibition on VP escape. If these studies suggest a role for PG in VP escape, then studies will be performed to determine if PG exert their effect through a cAMP or tissue solute mechanism. To better delineate the cellular mechanism of VP escape, the hydroosmotic response to VP will be studied in collecting tubules dissected from our rat models of VP escape. These experiments utilizing whole animal metabolic balance studies, clearance techniques, in vivo and in vitro biochemical analyses and tubular perfusion will be complementary in further clarifying the mechanism of VP escape.